1. Field of the Invention
This invention relates to a solid ultrasonic delay line, and more particularly it relates to a multiple reflection type solid ultrasonic delay line in which undesirable signals are remarkably attenuated and method of making the same.
2. Description of Prior Art
The conventional ultrasonic solid delay lines comprise a delay medium for propagating ultrasonic vibration such as glass, quartz, metal, ceramic, etc.; an input transducer for converting electric signals to ultrasonic vibration and an output transducer for converting ultrasonic vibration to an electric signal which are bonded on a desirable side surface of the delay medium, and which are polarised to the direction vertical to the ultrasonic vibration propagating direction and parallel to the major surfaces of the delay medium.
The transducers are made of piezo-electric materials such as quartz, lead titanate zirconate solid solution, etc.. These ultrasonic delay lines have been used for PAL colour television receiver, VTR etc.. The delay time is decided by the time required for propagating the ultrasonic vibration through the delay medium to reach the output transducer.
From the viewpoints of nondispersive propagation characteristics and compact structure, it is preferable to use a multiple reflection type solid ultrasonic delay line of a thin flat plate wherein the thickness of said flat plate in parallel to the direction for propagating ultrasonic vibration, is less than several times of the wavelength of the ultrasonic vibration. However, in the multiple reflection type solid ultrasonic delay lines, undesirable signals due to the diverging signals reaching the output transducer by paths other than intended are found.
In order to attenuate the undesirable signals, it has been proposed to provide an energy attenuation material on parts of the major surface of the delay medium other than the desired path of the ultrasonic vibration. However, it is disadvantageous to produce said delay line in mass production because energy attenuation materials must be provided on the major surfaces of the delay medium, of each unit produced. It has also been proposed to provide an energy attenuation material on portions of side surfaces other than desirable reflection elements for ultrasonic vibration on the side surfaces of a delay medium (hereinafter referred to as undesirable reflection elements).
The thickness (width of the side surface) of the delay medium is thin and the area for attenuating undesirable signals is narrow. Accordingly, it is hard to attenuate remarkably undesirable signals with the exception of matching the mechanical impedances between the energy attenuation material and the delay medium and providing enough thickness of the energy attenuation material.
It has been impractical to use such products because of the difficulty of the matching mechanical impedance, and of the method and accuracy for providing the products in practical manufacturing and because of the disadvantage that the energy attenuation material is easily peeled off.